Correctly handle UAS going away

src/ui/HDDisplay.cc

@@ -473,17 +473,18 @@ void HDDisplay::renderOverlay()
  */
 void HDDisplay::setActiveUAS(UASInterface* uas)
 {
-    if (!uas)
-        return;
     // Disconnect any previously connected active UAS
     if (this->uas != NULL) {
         removeSource(this->uas);
+        this->uas = NULL;
     }
 
+    if (uas) {
         // Now connect the new UAS
         addSource(uas);
         this->uas = uas;
     }
+}
 
 /**
  * Rotate a polygon around a point